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Recrystallization Ryan Huckaby CHEM 2123-540 2-13-2020

1 Ryan Huckaby Dr. Srinivasan 2423-530 2-6-2020 Recrystallization ABSTRACT Recrystallization is a purifying method used in chemistry to purify solutes. Recrystallization depends greatly on polarity as the solvents used for recrystallization must have a similar polarity to the solvent used to dissolve it. The cold solvent must have the opposite polarity of the solvent to get crystals as a product. After being purified one can use percent yield to determine how much of the chemical was purified and the identity of the solute. In the case of this experiment the resulting solute was acetanilide.

INTRODUCTION Just like chromatography, recrystallization is a technique used to purifying solid compounds. Recrystallization is the process of a crystalline solute is dissolved in a hot solvent and then returned to its solid state when cooled in a solvent by crystallizing. Crystal formation is a selective process. During recrystallization the solute is dissolved in a hot solvent destroying impurities. Once it has been heated, it is then cooled in a cold solvent selectively producing purer crystals. The size of the crystals determines how pure the compound is, as larger crystals are more pure than smaller crystals. The molecules of the impurities don’t fit as well causing less pure compounds to be smaller in size. The main factor in recrystallization is polarity. The solute should have maximum solubility in the hot solvent and minimum solubility in the cold solvent. The solvent used to dissolve the compound being used should have a similar polarity (like dissolves like). For compounds of higher polarity, the solvents typically used are ethanol and methanol, but nonionic compounds generally dissolve in water. Compounds of lower solubility are typically dissolved in diethyl ether and ethyl acetate.

2 Ryan Huckaby Dr. Srinivasan 2423-530 2-6-2020 MATERIALS & METHODS 

Digital Melting Point Apparatus

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Water

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Carbon (Charcoal)

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Acetanilide

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Capillary Tubes

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Hot Plate

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Powder Funnel

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Weighing Paper

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Unknown B

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Vacuum Tubing

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Thermometer

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Buchner Funnel

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Filter Paper

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Erlenmeyer Flask

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Filter Flask

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Beaker

The hot plate is a mechanism used to heat glassware and its contents. The melting point apparatus serves as a heater, thermometer, and observer as samples melt in the capillary tube. A vacuum is also used in the experiment. The vacuum system used consists of a filter flask, Buchner funnel, rubber tubing, and filter paper, to pull moisture out of the compound. Water MW: 18.02 g/mol Density: 1 g/cm3 Acetanilide MW: 135.17 g/mol Density: 1.22g/cm3

Carbon (Charcoal) MW: 12.011 g/mol Density: 2.1 g/cm3

3 Ryan Huckaby Dr. Srinivasan 2423-530 2-6-2020 PROCEDURE 1. A hot plate is used to heat water in a beaker until boiling. 2. Obtain 0.75 g of unknown B in an Erlenmeyer flask and dissolve in the hot solvent (water). 3. A small amount carbon charcoal was used to make the solution achromatic, by removing the pigment from the solution 4. The solution was then cooled to room temperature. (The thermometer was used to measure to room temperature 20 C). 5. Once the solution was room temperature, it was then placed in an ice bath to induce crystallization. 6. The crystals were washed with cold solvent. The crystals were then allowed to air dry and placed on a Buchner funnel and vacuum system to remove all moisture. 7. The crystals were weighed then placed in a melting point apparatus to determine its melting point

DIAGRAMS

4 Ryan Huckaby Dr. Srinivasan 2423-530 2-6-2020

RESULTS & DISCUSSION The crystal mass was calculated after being dried, and 0.339 g of crystal were obtained. Once the crystals were weighed the percent yield was then calculated to 45.44%. After calculating the percent yield a small amount of the crystals were put into a capillary tube, and into a melting apparatus. The melting point apparatus indicated that the unknown B compound was Acetanilide.

5 Ryan Huckaby Dr. Srinivasan 2423-530 2-6-2020 CALCULATIONS Unknown B obtained: .746 g Filter paper: 0.180 g Plastic: 14.423 g Filter paper, Plastic, and Crystals: 14.942 g 14.942−( 14.423 + .18 ) =.339 g .339 g ×100=45.44 % yield .746 g

SAFETY AND DISPOSAL Caution must be taken around the hot plate in as it is a hot surface. The capillary tubes were disposed in the glass disposal box. Remaining solvent was disposed in the sink. Glassware was cleaned and dried and put back.

CONCLUSION The unknown B was proven by melting point to be Acetanilide. There was a 45.44% percent yield. The percent yield indicated that about 45% of the solute was purified during the recrystallization process. REFERENCE Mohrig, J.R., Albert, D.G., Hofmeister, G.E., et al; Laboratory Techniques in Organic Chemistry, 4th ed., W.H. Freeman and Company, New York: 2014. Pp 221-235, Recrystallization

POST-LAB QUESTIONS

1. Describe the characteristics of a good recrystallization solvent. The solvent must be able to dissolve the solute at high temperatures and there should be no reaction between the two. 2. The solubility of a compound is 59 g per 100 mL in boiling methanol and 30 g per 100 mL in cold methanol, whereas its solubility in water is 7.2 g per 100 mL at 95 C

6 Ryan Huckaby Dr. Srinivasan 2423-530 2-6-2020 and 0.22 g per 100mL at 2 C. Which solvent would be better for recrystallization of the compound? Explain. Water would be a better solvent than methanol because it doesn’t have a significant difference in solubility at the high and low temperatures. 3. Explain how the rate of crystal growth can affect the purity of recrystallized compound. Slow crystal growth allows the selected molecules to form crystals easier. Faster crystal growth allows impurities to enter the crystals. 4. In what circumstances is it necessary to filter a hot recrystallization solution? A hot filtration is generally used in some crystallization, when a solid contains impurities that are insoluble in the crystallization solvent. 5. Why should a hot recrystallization solution be filtered by gravity rather than by vacuum filtration. Gravity filtration can be used to collect solid product. Keeping the solution hot prevent crystals from forming prematurely. This method is used to keep the insoluble impurities separated from the hot solution. 6. Low-melting solids often “oil out” of a recrystallization solution rather than crystallizing. If this were to happen, how would you change the recrystallization procedure to ensure good crystals? If the solution “oiled out” I would add more solvent to the solution or switch to a solvent with lower boiling point. I would also dissolve the oils in a small amount of diethyl ether or hexane and allow it evaporate in a fume hood....